Process for purifying molybdenum trioxide

ABSTRACT

Processes are described for preparing pure ammonium molybdate from impure roasted molybdenum concentrates. An aqueous solution of nitric acid and ammonium nitrate is contacted with impure molybdenum concentrate to solubilize a major portion of the impurities. The resulting molybdenum concentrate is digested in ammonium hydroxide under conditions that maximize iron precipitation and removal. The resulting ammonium molybdate solution is separated from the sludge and further purified by chelating cation exchange resin in the ammonium form.

The present invention relates to a process for preparing ammoniummolybdate of high purity from a source of molybdenum trioxide containingimpurities.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 3,860,419 to Weber, et al relates to a process forrecovering molybdenum from molybdenum concentrates by digesting theconcentrate in an aqueous nitric acid solution containing ammoniumnitrate, separating the solid phase from the liquid phase, and treatingthe solid phase with ammonium hydroxide to produce ammonium molybdate.In column 3, lines 9-16, the digesting solution is disclosed ascontaining substantially 1.0 to 4.0 moles per liter of free nitric acidand 0.5 to 2.0 moles per liter of ammonium nitrate. The weight ratio ofsolution to concentrate is substantially 1:1 to 3:1. In column 3, lines17-26, the preferred range of the solution is described as containingfrom about 2.0-2.3 moles per liter nitric acid and 1.5 to 2 moles perliter of ammonium nitrate.

Acid-leach processes such as U.S. Pat. No. 3,860,419 to Weber et al andU.S. Pat. No. 3,694,147 to Drobnick et al leave some calcium, magnesiumand other impurities in the roasted molybdenum concentrate. Impuritiesalso remain from water-leach processes such as described in U.S. Pat.No. 3,957,946 to Ronzio et al. The impurities contaminate the ammoniummolybdate solution and the crystalline ammonium molybdate product. InU.S. Pat. No. 3,957,946 chelating cation resin in hydrogen form is usedto remove residual contaminating ions from the ammonium molybdatesolution (column 6, lines 2-14).

SUMMARY OF THE INVENTION

The process of the present invention is effective in removingsubstantially all of the impurities from an impure technical grade ofroasted molybdenum concentrate. The present invention produces ammoniummolybdate having an extremely low impurity level since impurities areremoved according to a multistep process.

In accordance with the present invention, there is provided a method forpreparing ammonium molybdate having low impurity level from an impuremolybdenum oxide concentrate. First, the molybdenum oxide concentrate iscontacted with an aqueous solution comprising nitric acid and ammoniumnitrate at a suitable concentration to solubilize a major portion of theimpurities. This step is illustrated as LEACH in FIGS. 1 and 2. Inaccordance with the preferred embodiments, applicants' process mayutilize a lower ratio of aqueous leach solution per part of molybdenumoxide concentrate than one would expect from U.S. Pat. No. 3,860,419mentioned above. According to the same preferred embodiment, themolybdenum oxide concentrate is treated with from about 0.85 parts toabout 1 part by weight aqueous solution of nitric acid and ammoniumnitrate to remove a major portion of the impurities. Applicants believethat by using less leach solution per part of molybdenum oxideconcentrate than would be expected according to U.S. Pat. No. 3,860,419,more molybdenum values are retained in the concentrate so that animproved efficiency of the process can be achieved.

In accordance with the preferred embodiments, applicants' process mayutilize a higher nitric acid concentration than one would expect fromU.S. Pat. No. 3,860,419. Applicants believe that higher nitric acidconcentration provides better purification by solubilizing moreimpurities and by improving oxidation of Fe(II) to Fe(III). A furtherbenefit of the higher nitric acid concentration is improved oxidation ofresidue Mo(IV) to Mo(VI).

The next step is washing resulting molybdenum oxide concentrate followedby digesting the resulting molybdenum oxide concentrate with an aqueoussolution of ammonium hydroxide at suitable concentrations to solubilizesubstantially all of the molybdenum values present and to rendervirtually all of the iron values insoluble so that they may be separatedfrom the resulting ammonium molybdate solution. This step is illustratedas DIGESTION in FIGS. 1 and 2. According to a preferred embodiment ofthe present invention, certain conditions are employed to optimize ironprecipitation in the ammonium molybdate solution. According to anotherpreferred embodiment, precipitated iron is separated from the ammoniummolybdate solution after the ammonium hydroxide digestion and prior tosubsequent purification steps.

In the preferred operation of the invention, the ammonium molybdatesolution is passed through iminodiacetate chelating cation exchangeresin in the ammonium form to remove especially calcium and magnesium.Copper and certain other cations are also sorbed by the resin. This stepis identified as CATION EXCHANGE in the various embodiments in FIGS. 1and 2. Although the resin has very limited capacity for aluminum, it isused to advantage as long as possible. A preferred embodiment of theinvention is use of the ammonium form of the chelating resin.

In an alternate mode of operation, part or virtually all of the copperis first removed from ammonium molybdate solution by adding ammoniumsulfide. This step is identified as PRECIPITATION COPPER in FIGS. 1 and2. After separating the insoluble residues, the ammonium molybdatesolution then is passed through iminodiacetate chelating resin inammonium form. Preferred embodiments of the alternate method include:

A. Addition of ammonium sulfide is made after precipitated iron has beenseparated from ammonium molybdate solution.

B. Addition of ammonium sulfide is made in the absence of an oxidant,such as hydrogen peroxide.

C. The amount of added ammonium sulfide is such that a very lowconcentration of copper remains in ammonium molybdate solution. Additionof stoichiometric quantities of ammonium sulfide or of excess ammoniumsulfide is avoided.

Ammonium molybdate solution is concentrated to make crystalline ammoniummolybdate after the above ion exchange purification. While aluminum isadsorbed onto the resin, no further purification is needed. Oncealuminum passes through the resin, however, an aluminum-removal step isnecessary before crystallization. As a preferred embodiment of theinvention, ammonia is removed and the pH of the ammonium molybdatesolution is adjusted from about 6 to about 8 to convert substantiallyall of the soluble aluminum values to an insoluble aluminum hydroxide.This step is identified as PRECIPITATION ALUMINUM in FIGS. 1 and 2. Theinsoluble residue containing aluminum values is separated and theresulting ammonium molybdate solution is concentrated to form ammoniummolybdate having low impurities, especially aluminum, calcium, copper,magnesium, and iron. The crystallization step is identified asCRYSTALLIZATION in the drawings.

Because at least two additional separate steps are directed to theremoval of specific impurities, applicants believe that the presentprocess achieves a high purity product while minimizing loss ofmolybdenum values.

DRAWINGS

In the drawings,

FIG. 1 is a flow chart illustrating preferred process steps; and

FIG. 2 is a flow sheet of a detailed preferred embodiment.

DETAILED DESCRIPTION

The impure concentrate of molybdenum trioxide used as the startingsource is typically a technical grade of molybdenum oxide. Typically,such molybdenum oxide concentrates are produced by roasting molybdeniteconcentrates at elevated temperatures in the presence of excess air toconvert the predominant portion of the molybdenum sulfide to molybdenumtrioxide. The starting source typically contains about 7 to 15 percentby weight impurities. According to the preferred process of the presentinvention, it is desirable to make a very pure ammonium molybdateproduct. Preferably the ammonium molybdate product is about 99.95percent pure with impurities of copper, iron, aluminum, tin, calcium,magnesium, manganese, nickel, and lead each being present in an amountless than about 10 parts per million.

The molybdenum containing concentrate is contacted with an aqueoussolution comprising nitric acid and ammonium nitrate at suitableconcentrations to solubilize a portion of the impurities. Althoughmolybdenum trioxide is substantially insoluble in the above solutionwhich solubilizes the impurities, it is preferred to use from about 0.85parts to less than about one part by weight aqueous solution per part ofimpure molybdenum concentrate. It is theorized that using less leachsolution per part of concentrate results in less loss of the molybdenumvalues being leached from the concentrate. Preferably, the aqueoussolution comprises about 4.2 moles per liter of nitric acid and 1.75moles per liter of ammonium nitrate. Preferably the process is carriedout under suitable conditions so as to solubilize less than about 0.35percent molybdenum values from the concentrate and solubilize at least amajor portion of the impurities present in the concentrate. Typicalcontacting temperatures are from about 50° C. to 100° C. for periods oftime of about two hours.

It is next desirable to separate the solid molybdenum concentrate fromthe solution containing the impurities. Preferably, the molybdenumconcentrate is washed with water at a temperature of about 20° to about80° C. for about ten to thirty minutes. Typically, the molybdenumconcentrate may be separated from the wash solution by filtration,decantation, or any conventional liquid-solid separation.

Next, the molybdenum trioxide concentrate is digested with an aqueoussolution of ammonium hydroxide at suitable concentrations to solubilizesubstantially all of the molybdenum values present in the molybdenumconcentrate as ammonium molybdate. The ammoniacal leaching of theseparated molybdenum concentrate is performed by employing solutioncontaining from about 10 to about 25 percent by weight ammoniumhydroxide. Preferably, the solid content of the slurry is controlledwithin the range of from about 10 percent up to about 40 percent byweight. The slurry is digested to extract and solubilize substantiallyall of the molybdenum trioxide constituent in the separated molybdenumconcentrate. The preferred conditions are described:

1. A sufficient amount of hydrogen peroxide or other suitable oxidant isadded to the room temperature slurry of ammonium hydroxide andmolybdenum oxide. Digestion occurs for about 30 minutes.

2. The slurry is heated to 50° C.-60° C. and digested for about twohours. Most preferably the pH of the solution is maintained at a pHgreater than about 9.5.

In an alternate method the oxidant could be added during the heateddigestion, but more hydrogen peroxide is required as it decomposes onheating. In fact, one purpose of the heated digestion is to destroy theperoxide in the event that ammonium sulfide is added.

3. The digestion slurry is cooled to around room temperature.

4. The precipitated iron and the residual molybdenum concentrate whichcontains ammoniacal insoluble constituents such as silica are separatedto give an ammonium molybdate solution containing residual impuritiessuch as cations of copper, calcium, magnesium, aluminum and other. Theammonium molybdate solution is virtually iron-free.

Preferably, a sufficient amount of an oxidizing agent is added to theammonium molybdate solution to oxidize soluble iron values from a plustwo to a plus three state. Preferably the oxidizing agent is a materialsuch as hydrogen peroxide which does not contribute additionalimpurities to the solution. After the addition of hydrogen peroxide theslurry is digested about 30 minutes without external heating. pH is thenadjusted to >9.5 at room temperatures.

A purpose of the two hour digestion at 55° C. is to increase the size ofiron precipitates and sludge particles so as to facilitate theirseparation from ammonium molybdate solution. The pH of the solution isadjusted to greater than about 9.5 by adding ammonium hydroxide if thepH is lower than this value. The pH adjustment is such thatsubstantially all of the soluble iron values are converted to aninsoluble ferric hydroxide which may be conveniently removed byconventional liquid-solid separation techniques. The slurry is cooledbefore liquid-solid separation to maximize iron precipitation bydecreasing iron solubility. pH during liquid-solid separation is greaterthan about 9.5.

As preferred embodiments, precipitation of iron in ammonium molybdatesolution is optimized by adding oxidant, by maintaining pH around 9.5 orhigher, by digesting at around 55° C. for two hours, and by cooling toaround room temperature before liquid-solid separation. As anotherpreferred embodiment, the precipitated iron is separated from theammonium molybdate solution before any further purification. As anotherpreferred embodiment, pH during liquid-solid separation is greater than9.5. These measures promote the precipitation of iron and otherinsolubles from the ammonium molybdate liquor as a separate and distinctstep, and they improve iron removal.

In accordance with the principles of the present invention, it is nextdesirable to contact the ammonium molybdate solution with a chelatingcation exchange resin to remove additional cation impurities and recovera very pure ammonium molybdate solution. Typical chelate resins that canbe employed include:

N-(ar-vinylbenzyl) iminodiacetic acid, the polymers ofN-(ar-vinylbenzyl) aspartic acid, the polymers ofN-(a-methylar-vinylbenzyl)-iminodiacetic acid,N-(-methyl-ar-vinylbenzyl) aspartic acid, N-(ar-vinylbenzyl)-a,a-iminodiproprionic acid, and N-methyl-N-(ar-vinylbenzyl) aspartic acid.Preferred resins are Bayer Lewatit TP-207, Rohm and Haas IRC-718 andBio-Rad Chelex 100. Generally residence times of from about 2 to 7minutes are effective at temperatures of from about 20 to 50 degreescentigrade. The pH of the ammoniacal molybdate feed solution is fromabout 8.5 to about 10 and has a concentration of from about 150 to about250 grams of molybdenum per liter.

The cation exchange resins are typically purchased in the sodium form.To condition the resin prior to use, the resin is contacted with amineral acid such as sulfuric or hydrochloric acid to convert the resinto the hydrogen form. The acidified resin is washed and treated withammonium hydroxide to convert it to the ammonium salt form for receptionof the ammonium molybdate solution. In a similar manner, the resinloaded with cation impurities is regenerated by contacting the resinwith a mineral acid to strip the cation impurities and the resin isconditioned by contacting with aqueous ammonium hydroxide solution. Theconditioned resin is effective in sequestering calcium, magnesium,copper, manganese, nickel, aluminum, and other cations. The conditioningof the resin with ammonium hydroxide is preferable as compared withcontacting the molybdate solution with the acid form since the acid formlowers the pH of the molybdate solution.

As an alternate operational mode of the invention, it is sometimesdesirable to separate copper from the ammonium molybdate solution beforeion-exchange treatment. Ammonium sulfide is added to the ammoniummolybdate solution in an amount sufficient to convert substantially allof the soluble copper values present in the solution to an insolublecopper sulfide which in turn is separated from the solution as aninsoluble residue. Preferably ammonium sulfide is added incremetally andthe addition is stopped when a very small concentration of copperremains in solution. More preferably the amount used is just less thanabout the stoichiometric amount. The precipitated copper sulfide andother insolubles are removed.

It has been found that the addition of ammonium sulfide to an ammoniummolybdate solution containing iron or to a slurry of ammonium molybdateand iron precipitate results in the formation of soluble iron compounds.Also, it has been found that the addition of ammonium sulfide to anammonium molybdate solution containing iron and hydrogen peroxide or toa slurry of iron precipitate in ammonium molybdate solution containinghydrogen peroxide results in the formation of soluble iron compounds.Therefore, preferred embodiments include:

A. Maximized iron precipitation (as described earlier) with separationof iron precipitate from ammonium molybdate solution before ammoniumsulfide addition.

B. Addition of ammonium sulfide to ammonium molybdate solution that isiron-free or virtually iron-free.

C. Addition of ammonium sulfide to ammonium molybdate solution that isfree of hydrogen peroxide or other oxidant.

Small concentrations of copper are left in the ammonium molybdatesolution to avoid excess sulfide, which is not desirable for a number ofreasons: it has been found that excess sulfide causes a dirty grayprecipitate to form as long as the sulfide is present. Because aparticulate-free solution is needed for the ion-exchange column, thesulfide is destroyed by the addition of oxidant, some is converted tosulfur, which is removed by filtration, but some is converted tosulfate, which contaminates the product. These problems are avoided byusing less than stoichiometric amounts of sulfide.

As a preferred embodiment of this alternate mode of operation, ammoniumsulfide is added to nearly, but less than, stoichiometric amounts toavoid excess sulfide. Filtration of insoluble sulfides is facilitated byheating the ammonium molydbate/precipitate slurry for about 30 minutesat about 50° C. to aid particle growth and by then cooling to filter.After removing insoluble copper sulfide and other insolubles, theremaining ammonium molybdate solution is contacted with a chelatingcation exchange resin to remove calcium, magnesium, the small remainingconcentration of copper and additional cation impurities and to recovera very pure ammonium molybdate solution.

In accordance with the principles of the present invention, afteraluminum has started to pass through the ion exchange resin, it is nextdesirable to remove ammonia from the ammonium molybdate solution andadjust pH to a range of from about 6 to about 8 to convert substantiallyall of the soluble aluminum values to an insoluble aluminum hydroxide.Ammonia can be removed by ion exchange, evaporation or other technique.The insoluble aluminum hydroxide values are separated prior to furtherprocessing the resulting solution.

Typically, spray drying or a crystallization process may be utilized tofurther concentrate the molybdate values to obtain a purified source ofammonium molybdate.

EXAMPLE

Leach solution was prepared at 4.2 M HNO₃ and 1.75 M NH₄ NO₃. Technicalgrade molybdenum trioxide was slurried in the leach solution at a weightration of 0.95 parts leach to 1 part solids. The slurries were heated to75° C. to about 85° C. and maintained at the temperature for about 2hours. The slurries were stirred vigorously during the contacting. Themolybdenum oxide was then separated from the leach solution byfiltration. The percent of molybdeum lost to the leach was 0.32%. Themolybdic oxide was washed twice. Weight ratio of oxide to wash water wasabout 1:2. The washing involved slurrying the oxide in 70° C. water forten minutes and filtering to separate the oxide from the wash.

For the ammonical leaching the resulting slurry had a solids content ofabout 25 percent by weight. About one gallon 50% hydrogen peroxide wasadded to about 2000 gallons of technical grade molybdenumtrioxide/ammonium hydroxide slurry, and digestion without heatingoccurred for about 30 minutes. pH was then adjusted to about 9.9 byadding ammonium hydroxide. The slurry was then heated to 55° C. anddigested for two hours. pH was adjusted to 9.9 after cooling to around30° C., the insoluble sludge was separated from the resulting ammoniummolybdate solution. The ammonium molybdate solution with pH 9.0 wascontacted with Bayer Lewatit TP-207 resin in the ammonium formconditioned by contacting with a hydrochloric acid solution followed byan aqueous 7 percent ammonium hydroxide solution. Typical copperconcentration is 0.001 grams per liter to 0.02 grams per liter since anamount less than the stoichiometric amount of sulfide was previouslyused for copper removal. The feed of this example included 0.01 gramscalcium per liter and 0.15 grams magnesium per liter. A glass column sixinches in diameter and eight feet high with the Lewatit TP-207 resin wasused. Breakthrough occurred after 180 gallons of liquor was passed overone cubic foot of the resin. Residence time was five minutes and resincapacity at breakthrough was 10 grams calcium, 60 grams magnesium and 7grams of copper all per cubic foot. Next, ammonia is removed from thesolution until a pH of 7.2 is achieved. An aluminum hydroxideprecipitate is separated. Crystalline ammonium molybdate was thenproduced. Calcium, iron, manganese, magnesium, nickel, aluminum, copperwere all reduced to less than about 5 parts per million.

INDUSTRIAL APPLICABILITY

The present invention is useful for making a high purity ammoniummolybdate from impure technical grade molybdenum trioxide.

We claim:
 1. A method for preparing ammonium molybdate having lowimpurities aluminum, calcium, copper, magnesium, and iron from an impureconcentrate of molybdenum trioxide comprising contacting saidconcentrate with an aqueous solution comprising about 4.2 moles perliter of nitric acid, and about 1.75 moles per liter of ammonium nitrateto solubilize less than about 1.5 percent of the molybdenum values fromsaid concentrate and solubilize a major portion of said impurities,wherein during contacting from about 0.85 parts to less than about onepart by weight aqueous solution is present per part of impureconcentrate, washing the resulting concentrate, digesting said resultingconcentrate with an aqueous solution of ammonium hydroxide at suitableconcentrations to solubilize substantially all of the molybdenum valuespresent in said resulting concentrate as ammonium molybdate and form anammonium molybdate solution, adding ammonium sulfide to said ammoniummolybdate solution in an amount sufficient to convert substantially allof the soluble copper values present in said solution to an insolublecopper sulfide, separating said ammonium molybdate solution from aninsoluble residue, and contacting said resulting ammonium molybdatesolution with a chelate cation exchange resin in ammonium form, removingammonia to adjust the pH of said ammonium molybdate solution of fromabout 6 to about 8 to convert substantially all of the soluble aluminumvalues to an insoluble aluminum hydroxide, separating said resultingammonium molybdate solution from a resulting insoluble residue, andconcentrating said resulting ammonium molybdate to form ammoniummolybdate having low impurities, aluminum, calcium, copper, magensium,and iron.
 2. A process according to claim 1 wherein while saidmolybdenum concentrate is digested to form ammonium molybdate, ironprecipitation and removal are maximized by adding oxidizing agent tosaid ammonium molybdate solution to oxidize iron values from a plus twoto a plus three oxidation state, by adjusting pH of said solution togreater than about 9.5 by adding ammonium hydroxide, by digesting aroundone to three hours at around 50° C. to 60° C. to cause growth of ironprecipitate and sludge particles to facilitate more complete filtration,by cooling to around room temperature before liquid-solid separation todecrease iron solubility, by maintaining pH at >9.5 during liquid-solidseparation, and by separating iron precipitate and sludge from ammoniummolybdate before any further purification steps.
 3. A process accordingto claim 2 wherein the digestion at around 50° C. to 60° C. iscarried-out for two hours.
 4. A process according to claim 1 whereinammonium sulfide is added to ammonium molybdate solution in amountsnearly equal to, but less than, the stoichiometric amount needed forcomplete copper precipitation so that a small concentration of copperremains.
 5. A process according to claim 1 wherein ammonium sulfide isadded to ammonium molybdate that is free of iron and/or hydrogenperoxide or other oxidant.
 6. A process according to claim 1 wherein,following addition of ammonium sulfide, digestion occurs at around 55°C. for around 30 minutes to aid growth of the metal sulfide precipitatesto facilitate complete removal by filtration and wherein the slurry ofammonium molybdate and sulfide precipitate is cooled to around roomtemperature to filter.
 7. A process according to claim 1 wherein the ionexchange resin is a chelating iminodiacetate type used in the ammoniumform.
 8. A process according to claim 1 wherein aluminum is precipitatedand separated from ammonium molybdate solution by removing ammonia toadjust pH of said ammonium molybdate solution from about 6 to about 8and by cooling to around room temperature to convert substantially allof the aluminum to insoluble aluminum hydroxide and by separating theinsolubles from the ammonium molybdate solution.